NADA is a garbage bin for sail boats above 24 feet...you will do much better having a broker look at sold boats for current, real prices of boats similar to the ones you are looking at.
Better yet, in you price range and boat style, a simple sign on to their respective user's groups on yahoo or the net will bring very realistic information to your computer
I sold my 42' in august, was realistically priced, and we agreed to 8% off the asking price...but the buyer did not want a sea trial or survey, so I jumped. I told him everything that was wrong with the boat, but as he was knowledgeable of the boat and the maker, he was good with his decision.
Finding a true passage maker for <$35K may still be a challenge, unless it is a fixer upper, as passage maker suggests a certain build quality and level of equipment.
I would think 20% off is low on a well priced boat. 50% off on seller who is dreaming is not unheard of.
The boat is worth what you will pay for it. And sadly to use other people's offers, with no other information is asking for trouble. There is no shortcut to seeing the boat in person (pictures lie big time, and brokers often use sistership photos with out telling you), and to haul, survey and sea trial. It may be the best $500 you ever spend. And for sure, do not get attached to a boat, before you OWN her....this is a really bad idea, to convince yourself this is THE boat, when everything else tells you to run.